In abandoning a well, a cement plug may be installed in the wellbore. This plug may seal the wellbore against both upward and downward flow of fluid within the wellbore, past the plug. A wellbore that is to be abandoned may be lined with casing that is secured in place by an annular layer of cement. Over time, the cement may crack or degrade, thereby allowing fluids to leak through the cement from the surrounding formation. Thus, prior to abandoning the wellbore, segments of the cement may be removed to allow the cement plug to be formed and provide full rock-to-rock coverage spanning the full cross-section of the wellbore.
In a cased wellbore, a section mill may be used to remove sections of the casing and old cement, or an underreamer may be used to remove the old cement. The section mill may include blades or knives that are inserted into the wellbore in a retracted position and are thereafter expanded after reaching a desired location in the wellbore. The expanded knives cut into the casing and cement. The section mill may then be moved axially within the wellbore to cut along a length of the casing. Where an underreamer is used, the underreamer may be inserted into the wellbore, and may have cutter blocks in a retracted position. Upon reaching the desired depth or other location within the wellbore, the cutter blocks may be activated and expanded. When in an expanded position, the cutter blocks may cut, grind, or otherwise remove the old cement and potentially a portion of the previously-uncut rock, or “virgin formation” surrounding the old cement. If the wellbore is to be abandoned, a cement plug may then be formed in the wellbore where the old cement was removed.